1. Field of the Invention
This invention relates generally to marine seismic exploration and, more particularly, to a system for minimizing impulse response and coupling differences between hydrophones and geophones used in marine seismic exploration.
2. Description of the Related Art
In marine seismic exploration, a seismic survey ship is equipped with an energy source and a receiver for taking seismic profiles of an underwater land configuration. The act of taking profiles is often referred to as "shooting" due to the fact that explosive devices have been commonly used for many years as energy sources. The energy source is designed to produce compressional waves that propagate through the water and into the underwater land formation. As the compressional waves propagate through the land formation, they strike interfaces between formations, commonly referred to as strata, and reflect back through the earth and water to the receiver. The receiver typically converts the received waves into electrical signals which are then processed into an image that provides information about the structure of the subterranean formation.
Presently, one of the most common energy sources is an air gun that discharges air under very high pressure into the water. The discharged air forms a pulse which contains frequencies within the seismic range. Another energy source which is frequently used is a marine vibrator. Marine vibrators typically include a pneumatic or hydraulic actuator that causes an acoustic piston to vibrate at a range of selected frequencies. The vibrations of the acoustic vibrator produce pressure differentials in the water which generate seismic pulses free from spurious bubbles.
Just as different energy sources may be used to generate seismic waves in marine applications, different receivers may be used to detect reflected seismic waves. Typically, the receivers most commonly used in marine applications are referred to as hydrophones. Hydrophones convert pressure waves into electrical signals that are used for analog or digital processing. The most common type of hydrophone includes a piezoelectric element which converts physical signals, such as pressure, into electrical signals. Hydrophones are usually mounted on a long streamer which is towed behind the survey ship at depth of about 30 feet.
Alternatively, marine seismic techniques may use different types of receivers which detect different characteristics of the environment. For instance, in bottom-cable seismic recording, a combination of pressure sensitive transducers, such as hydrophones, and particle velocity transducers, such as geophones, can be deployed on the marine bottom. While geophones are typically used in land operations where metal spikes anchor the geophones to the ground to ensure fidelity of geophone motion to ground motion, geophones cannot be economically anchored in marine applications. Therefore, cylindrical, gimbal geophones are attached to the bottom-cable. After the cable is deployed from the seismic survey ship, the geophones simply lie in contact with the marine bottom where they fall. The gimbal mechanism inside the cylinder assures that the geophone element mounted therein is oriented vertically for proper operation.
As is obvious from the above discussion, a variety of seismic equipment and techniques may be used in an attempt to accurately plot the underwater land formation. Regardless of which technique or combination of equipment is used, each offers certain advantages and disadvantages when compared to one another. For instance, gathering seismic data with a towed streamer in areas populated with numerous obstacles, such as drilling and production platforms, can be difficult or even impossible because the streamer may strike one of the obstacles and tear loose from the towing vessel. Such an event represents an extremely costly loss.
By contrast, in bottom-cable seismic operations, no such difficulty exists because the cable is deployed in a fixed position on the water bottom. However, in the above-mentioned towing technique, the hydrophone being towed behind the survey ship is perfectly coupled to the water in which it is immersed. Unfortunately, in a bottom-cable operation, since there is no practical way to effectively couple a geophone to the marine bottom as it can be coupled to dry land, the geophone is not perfectly coupled to its environment. Therefore, the signals received from the imperfectly coupled geophone do not accurately reflect the quantity being measured.
The present invention is directed to overcoming, or at least minimizing, one or more of the problems set forth above.